1. Field of the Invention
The present invention relates to an air balance structure applied to a mechanism for compensating for the deadweight of movable members that move along a vertical axis in, for example, precision measuring instruments, precision machine tools and precision machining tools.
2. Description of the Related Art
Most precision instruments, machine tools and machining tools for which precise operation is required have movable members that move up and down along a vertical axis. Movable members that move along a vertical axis (hereinafter called vertical axis movable members), unlike those that move along the horizontal axis, ordinarily require a mechanism to compensate for the gravitational pull exerted by their own deadweight. As a mechanism of this sort either a counterbalance system or an air balance system is generally used to compensate for the force of gravity. Of these two systems, the counterbalance system requires the installation of a weight that is the same weight as the movable member as the counterbalance. This arrangement has two disadvantages: It hinders efforts to make the entire apparatus compact, and it unavoidably increases the overall apparatus weight by the equivalent of the weight of the movable member.
By contrast, the air balance deadweight compensation mechanism, although it does not compensate for the force of gravity acting on the movable member with an equivalent weight, does require a seal means that contains pressurized air that generates a balancing force. Moreover, if high-precision operation is required of the movable member, the seal must be a non-contact seal, that is, a contactless seal.
FIGS. 1 and 2 are diagrams illustrating an air balance structure used in a contactless seal-type air balance used conventionally (that is, a structure for compensating for the force of gravity with an air balance system), with FIG. 1 showing a general external view of the air balance structure and FIG. 2 showing a schematic cross-sectional view of the structure along a line A-A shown in FIG. 1.
In FIG. 1, an air balance structure designated in its entirety by reference numeral 1, includes a stationary member 10 and a movable member 20. The movable member 20 has a substantially square cylinder shape, with the stationary member 10 inserted through the open bottom of the movable member 20. Reference numeral 14 designates a gap between the outside surface of the stationary member 10 and the inside surface of the square cylinder-shaped movable member 20, having a width great enough (for example, several millimeters) to permit the free passage of air.
The movable member 20 is supported so as to be movable up and down (in the direction of gravity) in a straight line with respect to the stationary member 10 by a vertical axis straight line drive mechanism, not shown. The air balance mechanism is provided in order to reduce the weight load on this line drive mechanism. As can be easily understood by an examination of FIG. 2, a cylindrical concavity open toward the top is formed in the stationary member 10. The movable member 20 is provided with a bridge 21, from which a piston 22 depends. Reference numeral 23 designates a shaft connecting the bridge 21 and the piston 22.
The piston 22 has an outer diameter that just fits the cylindrical concavity, so as to form an air balance chamber with the bottom of the concavity. A pressure adjusting device 30 supplies pressure-adjusted, compressed air to this air balance chamber 11 via pipes 31, 12. The air is pressurized to a value that is just capable of offsetting the load weight exerted by the movable member 20. Thus, the conventional air balance structure pushes the piston 22 up and generates a balancing force that cancels out the deadweight of the movable member 20 by pressurizing the air balance chamber 11 with air adjusted to an appropriate pressure with the pressure adjusting device 30, and therefore requires high-performance air sealing.
Although for purposes of illustration the gap 13 formed between the inside wall of the concavity and the side of the piston 22 is shown as being approximately the same as the gap 14 described above, in actuality the gap 13 is smaller than the gap 14 and is in general 20 microns or less, so as to minimize the leakage of air from the interior of the air balance chamber 11. If the gap 13 that seals the air in is large, then the leakage of air from the air balance chamber 11 increases as well, which not only increases the consumption of air but also the flow of air, and the flow of air generates vibrations that interfere with the precise placement of the vertical axis. It is for this reason, then, that the gap 13 is usually held to 20 microns or less, but it should also be noted that flow of air through this tiny gap 13 is not linearly correlated with the size of the gap 13, and thus even a small change of several microns in the size of the gap 13 can cause large changes in the flow of air.
Therefore, not only must the gap 13 be small, but the vertical movement of the piston 22 must be steady. In other words, in order to achieve a steady, stable air balance at all times as the movable member 20 moves, the cylindrical inside wall of the air balance chamber 11 must extend accurately in the direction of movement of the movable member 20. In addition, if there are localized areas of excessive narrowness in the gap 13, then friction will arise between the piston 22 and the inside wall of the cylinder, thus interfering with the smooth movement of the piston 22 and therefore of the movable member 20.
Accordingly, the dimensional accuracy and the assembly accuracy required of the components that comprise the air balance are on the order of microns. Consequently, although there is little concern that the air balance mechanism adds weight, it does require a structure that seals in pressurized air, thus complicating the entire mechanism and increasing the manufacturing and machining costs of the components that are used. No publications disclosing a technology that remedies the defects of the air balance mechanism have been found.
In order to drive the movable member along the vertical axis with high precision, it is important to keep the balancing force that cancels out the weight load steady. However, as described above, with the conventional air balance structure, in order to keep the gap (that is, the seal) between the piston and the inside wall of the cylinder, the machining ands the assembly of the air balance structure components must be performed with high precision. As a result, manufacturing costs naturally increase.